1. Field of the Invention
This invention relates to a method for the prevention of Fusarium disease by the use of microorganisms that have the ability to decompose or detoxify fusaric acid.
2. Description of the Prior Art
It is known that the genus Fusarium contains species that can cause diseases of wither and blight that occur during the growth of plants, such as vine-splitting of members of the gourd family such as cucumbers, watermelons, melons, etc., tomato blight, half-wither disease of eggplants, yellow blight of strawberries, dry rot of devil's foot root, spring blight of grass, etc., by infecting the plants. These kinds of infectious diseases are caused by species of Fusarium named Fusarium oxysporum, Fusarium moniliform, and a large number of other species. These fungi contaminate the soil, and are absorbed into the plant from the soil via the xylem. It is thought that the cause of the withering of plants is the fusaric acid that is produced by the metabolism of these fungi. Fusaric acid is produced by almost all species that belong to the genus Fusarium (including the two species mentioned above) that are plant pathogens. This fusaric acid acts as a non-specific toxin, and it damages not only the host plants but also other kinds of plants. Fusaric acid causes an increase in the permeability of plant protoplasmic membranes with respect to tissue fluids that contain Ca.sup.2+ , K.sup.+, Na.sup.+, or other cations, or that contain various kinds of amino acids, which increase causes exudation of the tissue fluids to surfaces such as those of the leaves of the plant. This exudate dries, and the surfaces of the plants develop high osmotic pressure, which increases the evaporation of water still more. As a result, the plant dies of wilting.
Chemical agents that are used to prevent Fusarium infections include, for example, the o-phenylenediamine derivative with the structure shown below, with the trade name of Topjin-M, which is used as a soil fungicide. ##STR1## This agent kills the fungi of the genus Fusarium that live in the soil, but if it merely scattered on the surface of the soil, it is not effective, so it is necessary to mix it into the soil before sowing or planting. This operation requires much labor. There is also the disadvantage that once the symptoms of the infection have appeared, this agent has almost no effect on stopping the progress of the disorder.
There is a variety of tomato that is resistant to infections caused by species of Fusarium. It has been found that, in the tissues of this variety of tomato, as shown below, fusaric acid (I) is metabolized and decomposed to form N-methylfusaric acid amide (II). The decomposition of fusaric acid is attained effectively among varieties of tomatoes that have strong resistance to Fusarium infections. ##STR2## If fusaric acid is made nontoxic in this way by decomposition or chemical modification, it might be possible to prevent plants from losing water, which damages the metabolism of water components. For example, if a microorganism was found that decomposed or detoxified fusaric acid, it could be used as an effective method for the prevention of Fusarium diseases. In addition, if the gene that controls the decomposition or detoxification of fusaric acid could be isolated, it might be possible to obtain microorganisms potent in the causing of this effect, and these microorganisms would be useful in the prevention of such plant diseases.